This invention relates to an improved method of operating a spark ignition internal combustion engine with unleaded gasoline containing methylcyclopentadienyl manganese tricarbonyl (MMT) as a primary antiknock compound and an effective co-antiknock compound. Specifically, the invention relates to a method of operating said engine with unleaded gasoline containing MMT and a co-antiknock compound which does not react with metals contained in gasoline distribution systems and automobile storage tanks.
MMT, which was developed by Ethyl Corporation, is the only proven antiknock for use in unleaded gasolines. Until recently MMT was widely employed in unleaded commercial fuels. Because of its widespread use, the Environmental Protection Agency specified in January 1977 that 1/8 g Mn/gal as MMT must be used in gasoline by the auto companies to qualify their 1979 models for emission standards. Car manufacturers were concerned about this requirement as it would make it more difficult to meet future exhaust emission standards. Accordingly, it was suggested that the use of MMT in gasoline be limited. A level of 1/8 g Mn/gal can raise hydrocarbon emissions somewhat, making it more difficult to meet the present hydrocarbon emission limit of 0.41 g/mile. In June 1977, Ethyl Corporation recommended that the maximum MMT content of gasoline be limited to 1/16 g Mn/gal instead of 1/8 g/gal, contending that this lower Mn level does not effect "engine out" hydrocarbon emissions (i.e., prior to any exhaust catalyst). In July 1977, the California Air Resources Board banned the use of MMT in California.
In September, 1977 the EPA rescinded the requirement that auto manufacturers use MMT in gasolines used to certify 1979 model cars for emission standards. This action was based on the Clean Air Act amendments of 1977, which banned the use of MMT after Sept. 15, 1978 unless the EPA determined that a specified concentration of the additive does not contribute to the failure of vehicles to meet emission standards. In justifying the changes, the EPA reasoned that there is no need for MMT in qualification gasolines, since the 1979 model cars would operate in the field either on MMT free gasolines or on ones containing no more than a level of MMT that had been determined not to adversely affect emissions. Currently MMT is banned from unleaded gasoline in the United States; however, it is presently used extensively in unleaded fuels in Canada. It is likely that at some future time MMT may again be cleared for use in unleaded gasolines in the United States at up to a concentration of 1/32 g Mn/gal.
Since MMT is limited in any case to low concentrations in gasolines because of potential side-effects (e.g., spark plug fouling, valve burning, higher hydrocarbon emissions, and catalyst plugging), ways of enhancing the activity of MMT are highly desirable. In the late Fifties, it was discovered that some oxygenated organic compounds (e.g., beta-diketones and carboxylic acids), which are not antiknocks in themselves and therefore are referred to as co-antiknocks, were effective in enhancing the octane quality of gasolines containing tetraethyllead (TEL) or MMT. Since there was little interest at that time in metal-organic antiknocks other than TEL, little effort was devoted to finding MMT co-antiknock compounds.
It is known from Canadian Pat. No. 707,695 which is incorporated herein by reference that when certain diketones are added in small amounts to high octane number motor fuels containing, as primary antiknock agents, e.g. tetraethyl lead or the class of manganese compounds disclosed in U.S. Pat. No. 2,818,417, that the octane number of the fuel compositions is unexpectedly increased even further by as much as 2.5 octane numbers, even though in the absence of the primary antiknock agent substantially no octane number benefit is obtained by such additions. Effective diketones are said to be those having the formula: ##STR1## where, n is a whole number from 1 to 2 inclusively. The symbol R denotes a monovalent hydrocarbly radical having from 1 to 10 carbon atoms selected from the group consisting of alkyl, aryl, cycloalkyl, alkylaryl, and arylalkyl. This patent further discloses that when MMT is used as the primary antiknock agent, it is preferred to employ in the concentrate an amount of diketone corresponding to from about 60 to about 110 parts by weight of carbonyl per part of manganese metal, with a preferred ratio of from about 80 to 90.
It is also known from U.S. Pat. No. 3,261,674 that MMT exhibits a precipitate-forming decomposition reaction when gasolines containing same are exposed to sunlight. This reaction can be inhibited by incorporating a beta-diketone in the MMT, thereby enabling retention of the MMT in solution. From 1 to 3 mols of a hydrocabon oil soluble beta-diketone will inhibit 1 mol of MMT antiknock agent against a precipitate-forming decomposition reaction resulting from exposure to radiation such as sunlight. Concentrations greater than 3 moles of beta-diketone inhibitor to one mole of MMT were not recommended since the added inhibition protection is insufficient to warrant the increased cost of using such larger amounts of inhibitor.
An investigation of many of the beta-diketone co-antiknock agents disclosed in the prior art revealed that not all beta-diketones encompassed by the disclosures are effective as co-antiknock agents. Furthermore, it was discovered that most of the beta-diketones reacted with wet rust present in gasoline distribution systems resulting in loss of co-antiknock activity. It was also discovered that many of the beta-diketones reacted with terneplate (90% lead --10% tin alloy) which is used as internal surface coating for vehicle fuel tanks. Accordingly, it is an object of this invention to provide a method of operating a spark ignition internal combustion engine by providing an effective co-antiknock compound for unleaded gasoline containing MMT. Such a compound must not lose its co-antiknock effectiveness in the presence of wet rusted steel contained in gasoline distribution systems or in the presence of terneplate contained in the internal surface coating of automobile fuel tanks.